The present invention relates generally to optical tomography, and more specifically to a system and method of detecting acousto-photonic emissions in optically turbid media.
In recent years, optical imaging techniques have been increasingly employed in the field of biomedical imaging. Optical imaging yields important advantages in the biomedical imaging field due to its ability to locate objects and/or abnormalities in biological tissue without requiring the use of ionizing radiation. For example, optical imaging techniques have been used to detect breast cancer, to perform functional imaging of the brain and for stroke differentiation, to determine the health of fetuses, and to perform mechanical and optical tissue characterizations. Because the optical properties of diseased biological tissue typically vary from that of healthy tissue, optical imaging techniques can detect tissue abnormalities such as breast cancer based on the optical differences of the diseased and healthy tissue. Such use of optical imaging has drawbacks, however, because biological tissue is a turbid medium, and laser light typically used in optical imaging techniques generally undergoes a high degree of scattering within turbid media. As a result, good spatial resolution using optical imaging techniques in biomedical imaging has been difficult to achieve.
More recently, optical imaging has been employed in conjunction with ultrasonic techniques to improve spatial resolution in biomedical imaging. Whereas laser light is generally highly scattered within biological tissue, ultrasonic waves generally scatter much less readily within such tissue and can therefore provide good spatial resolution even at depth. Biomedical imaging using a combination of optical imaging and ultrasonic techniques is known by various names including acousto-photonic imaging, ultrasound tagging of light, acousto-optic tomography, acousto-optic imaging, and ultrasound-modulated optical tomography.
For example, in a typical mode of operation, an ultrasonic wave is propagated within a turbid medium of biological tissue, and laser light is sent through the tissue where it is modulated by the ultrasonic wave. There are three primary mechanisms for ultrasonic modulation of the laser light. In a first mechanism, the ultrasonic wave generates a pressure variation in the medium of interest to induce a density change in the medium. The optical absorption, the scattering coefficient, and the index of refraction of the medium vary with the change in density, and the light is modulated in response to these parameter changes. In a second mechanism, the ultrasonic wave generates particle displacement within the medium, thereby causing optical path lengths to change. These optical path length changes cause speckles to form, which subsequently lead to changes in the intensity of the light. In a third mechanism, the ultrasonic wave acts like a phonon, and the phonons interact with the photons from the laser, causing a Doppler shift of the optical frequency by the ultrasonic frequency. The optical detector operates as a heterodyning device between the Doppler shifted light and the non-shifted light to produce a signal of the ultrasonic frequency.
Next, the ultrasound-modulated light emitted from the tissue is detected, and the detected signal is analyzed to determine the location(s) of abnormalities within the tissue. Because the interaction region of the ultrasonic wave and the laser light is generally defined by the dimensions of the ultrasonic beam and/or the size of the acoustic focal region, and because the signals detected at the frequency of the ultrasonic wave correspond only to the light that has passed through the ultrasonic beam, spatial resolution in biomedical imaging can be significantly increased.
Various methods have been employed to detect emissions of ultrasound-modulated laser light in acousto-photonic imaging. For example, ultrasound-modulated laser light may be detected using a single high-speed detector such as a photo-multiplier tube (PMT) detector or an avalanche photo-diode (APD) detector. According to one detection method using a single detector, the mutual interference of partially coherent laser light produces a speckle pattern, and the single detector may have a detection aperture operative to receive either a single speckle or multiple speckles for subsequent analysis. The single speckle detection method, however, operates on very low levels of light, and therefore typically provides a low signal-to-noise ratio (SNR). Further, the multiple speckle detection method typically results in a reduced modulation depth.
Ultrasound-modulated laser light may also be detected using a charge-coupled device (CCD) array. According to one detection method using a CCD array, the size of a speckle is adjusted for approximately matching the size of a single pixel of the CCD array. Next, the modulation amplitude at each pixel is measured, and the measured modulation amplitudes are summed. Such a detection method typically results in an increased SNR. The ultrasound-modulated laser light may also be detected by measuring changes in the modulation depth on the CCD array.
Each one of the above-described methods of detecting emissions of ultrasound-modulated laser light has drawbacks, however, because the signals detected by such methods are typically very weak. As a result, the sensitivity of these detection methods, particularly in biomedical imaging, is typically very low. Although spatial integration may theoretically be employed to provide a stronger signal for increased sensitivity, the randomness introduced by speckle patterns generally reduces the effectiveness of spatial integration. Temporal integration may also be ineffective at increasing sensitivity if the biological tissue of interest undergoes any movement during the acousto-photonic imaging process.
It would therefore be desirable to have an improved system and method of detecting acousto-photonic emissions in optically turbid media such as biological tissue. Such an improved system and method would provide increased detection sensitivity, while avoiding the drawbacks of the above-described conventional acousto-photonic emission detection techniques.